1. Field of the Invention
The present invention relates to a linear motor which utilizes a magnetic field to support the movable member and incorporates a voice coil as the linear motor to induce a non-contact motion to the movable member.
2. Description of the Prior Art
Linear type actuators find particular utility in disk drives where they are used to position magnetic and/or optical type heads above a track of the disk to perform either a read or a write function. In such an application, the speed of positioning the head is crucial and once the head is positioned at the desired position the maintaining of the head at that position without high frequency vibration and head runaway is important. Head runaway is caused after an external shock disturbs the head from its tracking position. The speed of positioning a linear actuator is influenced by friction losses. Various types of mechanical and fluid actuators have been devised using, for example, either oil or air to freely support the movable member.
A linear motor of particular interest is disclosed in U.S. Pat. No. 4,689,529 entitled "Linear Stepping Motor" by T. Higuichi. The motor of that patent utilizes a magnetic field for suspending the movable member in a floating state. Magnetic poles are positioned on the movable member and/or the support member and are actuated in a step fashion in accordance with the desired positioning of the movable member. Because the motor is a stepping motor, a plurality of evenly spaced pole faces are required. The motion of the stepping motor is such that it moves from one pole pair to the next pole pair under the influence of a control signal. Additionally, there is provided a displacement sensor which measures the gap between the movable member and the stationary member to provide a displacement signal the magnitude of which indicates the distance of the movable member from a desired support or levitated position. The displacement signal is used to increase and/or decrease the strength of the magnetic field used to suspend the movable member. Traditionally, stepping motors do not provide a smooth transition from a start position to the desired end position. Therefore, where smoothness of motion is a requirement, something other than a stepping motor is required. The present invention fills the smoothness requirement.
Another patent of particular interest is U.S. Pat. No. 4,334,207, entitled "Linear Displacement Transducer" by C.S. Bill et al. In the transducer of that patent the linear displacement member is supported by fixed magnetic fields in a floating position. The floating position maintains a substantially constant gap. Windings are positioned on the supporting member and are in one embodiment positioned on the moving member. The windings receive an AC signal which causes Eddy currents that modify the flux between the windings causing in turn the movable member to move bilinearly within the supporting magnetic fields.
The present invention is directed towards achieving an increase in the smoothness of the positioning of the linear movable member and in the speed at which the movable member reaches its final destination and, additionally, in maintaining a high degree of physical rigidity once the movable member has reached the desired position.